As a result of progress in digital communication and signal processing, a high-speed and high-accuracy A/D converter (ADC) is needed to meet rapidly expanding communication bandwidth and higher speed processing.
For example, an ADC disclosed in IEEE Journal of Solid-State Circuits, Vol. 36, No. 2, February 2001 includes two conversion systems, which are a delta-sigma (ΔΣ) processing system and a cyclic processing system. The delta-sigma processing system has higher accuracy than the cyclic processing system. The cyclic processing system has higher conversion speed than the delta-sigma processing system. In a device incorporating such an ADC, the delta-sigma processing system is used to convert most significant-side bits, which need be converted accurately, and the cyclic processing system is used to convert least significant-side bits, which need be converted speedily.
The ADC disclosed in the above-referred document includes a capacitive D/A converter (DAC) for a feedback of quantized results. This capacitive DAC includes a first DAC, which feeds back quantized results to a circuit of delta-sigma processing, as well as a second DAC and a third DAC, which feed back the quantized results to circuits of cyclic processing. A capacitor of the first DAC is used as not only a sampling capacitor in the delta-sigma processing but also an integration capacitor in the cyclic processing. Capacitors of the second and third DACs are used as sampling capacitors in the cyclic processing.
For this reason, when leak currents are generated in the DACs and in switches, which turn on and off in the sampling operation and the integration operation, accuracies of sampling and feedback operations are lowered thereby lowering the accuracy of A/D conversion. The leak current of a switch, which is formed of a MOS or MOSs, for example, increases generally as a semiconductor device is manufactured with highly micronized technology. As a result, the leak current affects more on the accuracy of A/D conversion as more micronized manufacturing technology is used.